Chromatic dispersion optical time domain reflectometry on submarine communication cables using coherent optical transponders

A transceiver, comprising:

• an optical source configured to provide an optical signal into a fiber optic cable having one or more optical repeaters forming one or more fiber optic spans;
• a modulator receiving the optical signal, the modulator configured to encode data into a plurality of subcarriers in the optical signal;
• a transmitter module having circuitry configured to receive data to be encoded into one or more subcarrier of the plurality of subcarriers of the optical signal, the circuitry including at least one driver circuit supplying drive signals to the modulator to cause the modulator to encode data into a first subcarrier having a first frequency band, and the circuitry configured to cause the modulator to generate a first pulse at a first instance in time and a second pulse at a second instance in time, each of the first pulse and the second pulse comprising a test subcarrier having a test frequency band and an optical tone outside of the test frequency band, into the optical signal, the test frequency band and the first frequency band being different;
• a coherent receiver operable to receive the optical signal from the fiber optic cable, the optical signal having a filter spectrum, the filter spectrum being a portion of the optical signal having at least the test subcarrier and the optical tone, the test subcarrier having one or more comb lines;
• and
• a digital signal processor operable to:
• determine a first phase for each of the one or more comb lines of a first pulse reflection in the filter spectrum corresponding to the first pulse and received at a third instance of time, the first phase being determined using the optical tone of the first pulse reflection as a first reference, the third instance in time being between the first instance in time and the second instance in time;
• determine a second phase for each of the one or more comb lines of a second pulse reflection in the filter spectrum corresponding to the first pulse and received at a fourth instance of time, the second phase being determined using the optical tone of the second pulse reflection as a second reference, the fourth instance being between the third instance and the second instance;
• determine a first dispersion across a first fiber optic span of the one or more fiber optic spans based at least in part on the first phase and the second phase;
• determine a third phase for each of the one or more comb lines of a third pulse reflection in the filter spectrum corresponding to the second pulse and received at a fifth instance of time, the third phase being determined using the optical tone of the third pulse reflection as a third reference, the fifth instance being after the second instance;
• determine a fourth phase for each of the one or more comb lines of a fourth pulse reflection in the filter spectrum corresponding to the second pulse and received at a sixth instance of time, the fourth phase being determined using the optical tone of the fourth pulse reflection as a fourth reference, the sixth instance being after the fifth instance;
• determine a second dispersion across the first fiber optic span based at least in part on the third phase and the fourth phase;
• and
• determine a change to a seismic pressure within the first fiber optic span based on the first dispersion and the second dispersion across the first fiber optic span.